Additives in silver paste improve the mechanical and thermal stability of thermoelectric modules composed of n-type half-Heusler and p-type oxide materials

Thermoelectric modules composed of n-type half-Heusler (HH: Ti_0.75Hf_0.25NiSn) and p-type oxide (OX: Ca_2.7Bi_0.3Co₄O₉) materials have been prepared using silver (Ag)-based pastes to form junctions. The maximum power output P_max of the modules depends on the ratio of the cross-sectional area between the HH and the OX materials because of their different electrical and thermal conduction properties. The highest P_max value is obtained 1.7 W for the 16 n- and p-type thermoelectric pairs at the cross-sectional area ratio of n:p = 2:5. The maximum conversion efficiency η _max is 1.1 % at about 500 K of the temperature difference between the hot and the cold sides of the module. Single-junction thermoelectric devices composed of either HH or OX materials and Ag sheet electrodes were prepared using the Ag paste mixed with silver oxide (Ag₂O) or OX (same composition as the p-type material) powders, respectively, to compare the electrical resistance and joining strength. The additives in the Ag paste affected the electrical contact resistance and the joining strength at the junction between the thermoelectric materials and the Ag sheets. The increase in electrical contact resistance due to thermal shock was suppressed by the additives for both n- and p-type devices. Although the addition of Ag₂O powder enhanced the joining strength between the HH material and the Ag sheet before thermal shock, this effect is not clear after thermal shock. Moreover, the additives improve the thermal durability of the HH/OX module against the hot-side temperature above 673 K. The degradation of the module is related to the electrical contact resistance at the junctions.